I figured out first problem... the disappearing thing
Its quite an easy solution
_root.coin._x = Math.random()*500+50;
_root.coin._y = Math.random()*500+50;
that means your coin will re appear at 550 on the x and y axis when your file is only 500x400...
_root.coin._x = Math.random()*400;
_root.coin._y = Math.random()*500;